First of all, and there are two lines that keep bugging me - Jorak Uln's trick question "A True Sith never dies" and Kreia's scolding that "The Sith are a belief."

In the first line, it should be noted that Jorak was stating that a true practitioner of Sith beliefs does not believe in the fact that will die. He considers himself all-powerful, all-conquering and immortal. Jorak was not referring to the "True Sith".

In the second line, Kreia is referring to Revan's Sith. Revan never encountered or hired the original Sith in his crusade against the Republic, like Exar Kun. But he shared the evil intensity, the hate against Jedi that was present in the only slice of True Sith the Republic had seen - Naga Sadow. Since there was not much knowledge about Ludo Kressh or his supporters, it was a general assumption that Sith are evil beasts, worshippers of the Dark Side and against the Jedi and the Republic. Exar Kun and Darth Revan took up the title of The Sith to represent their faction, so that they could forward the dark-sided legacy of the Sith. Moreover, the Sith were the only major dark-sided faction the Republic knew of.

Kreia says that the Revan and Exar followed the Sith ideology, but technically were not Sith themselves. The real Sith, are the true Sith and they are more than the ideology that Revan followed.